Minimum Acceptable Bolt Pretension to Avoid Loosing Clamp

[msii]

Hi Guys,

I' m struggling with an issue regarding bolt minimum acceptable pretension. I'm designing a clamp for aluminium tube (DN 50)and I'm using finite element analysis to check bolt loads and loads on tube. Bolts are M5 and if I follow available bolt pretension suggested by different manufacturers and charts Aluminium tube would collapse due to high pretension even if I increase thickness of tube. In this situation I want to know what minimum acceptable bolt pretension is to avoid loosing clamp and keeping clamp in its place.
Schematic shape of clamp is presented on below website:

http://www.pipingtech.com/products/ptpcat/support/clamp/fig60.htm

please note "C" value should be 2mm.


Thank so much

 

[MikeHalloran]

That depends on how much force and/or vibration and/or torque is trying to slide the clamp along or rotate the clamp relative to the tube.

What, exactly, is the purpose of the clamp?

What is the environment (fluid, temperature, pressure, velocity) inside and outside the tube?





Mike Halloran
Pembroke Pines, FL, USA

 

[dik]

Can you use a Belleville washer?

Dik

 

[msii]

Washer cannot be used. Also, the purpose of clamp is supporting of a piping system. water is inside the tubes and temperature is amb. with low pressure. no possibility to using rubber for inside of clamp.

 

[TehMightyEngineer]

If you can't pretension normally I wouldn't even consider it as being effective. Locktite it, pin it, or safety wire it and be done with it.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[ENGINEER92]

I have never messed with aluminum clamp connection before, but I know for steel pipes it is not uncommon for a pipe to experience yielding before it reaches the pretension that the bolt is capable of.

What is the nominal sliding force you have to resist and is their any nominal twisting force the clamp connection has to resist?

 

[dik]

Locktite Red works really well... and I've never had anything using it ever loosen... even with the firm application of a wrench...

Check with the manufacturer to see if OK with aluminum... also aluminum has a higher coef of linear expansion.

Do you have issues with galvanic corrosion?

Dik

 

[drawoh]

mosii,

The last time I designed a tube structure that had clamping bolts, I specified that the holes were to be drilled through plugs welded through the tube. That way, the bolts clamped on solid material and and could be tightened down hard. Your next choice is to not drill through the aluminium tube. Weld a flange on one side and drill through that.

Bolts come loose because they are not tightened down hard. You need your design to allow you tighten bolts down hard.

--
JHG

 

[dik]

and if aluminium material, careful with damaging the bearing surface...

Dik

 

[WARose]

[blue] (mosii) [/blue]

In this situation I want to know what minimum acceptable bolt pretension is to avoid loosing clamp and keeping clamp in its place.

If you can't clamp to the yield strength of the material......then a minimum to use is 15%. That is recommended in ACI 351.3R-04 (but acknowledges that it may not be adequate for high vibration situations). That should get it "snug" tight......but that is for machinery hold-down....not sure if this would be adequate for sealing up a pipe that has water in it.

 

[MikeHalloran]

Unless crushing the tube is a requirement, that's a terrible clamp design.

To just hold the tube up, a simple loop of rope around the tube and some stationary object would do just fine.

If the clamp needs to resist thrust, you need to find out how much.

If the tube will be vibrating because of pressure fluctuations, you might need something like the stackable plastic clamps used on machine tools' hydraulic tubing.




Mike Halloran
Pembroke Pines, FL, USA

 

[dik]

If you can't clamp to the yield strength of the material.....

The expression, "Tighten 'er till she cracks, then, back off half a turn", comes to mind.

Dik